Current HIV vaccine candidates elicit reasonably potent cellular immune responses, but only low levels of neutralizing antibodies. Such CTL based vaccines (e.g., those based on DNA immunization) may not prevent infection, but can have a beneficial effect on disease course. In contrast, passively infused antibodies can provide complete protection, but must be administered in doses that result in serum antibody levels much higher than can be generated by current immunization strategies. We are using the SIV macaque model to study the protective effect of CTL based vaccines on SIV infection. In particular, we are studying the ability of such vaccines to affect plasma viral load, memory CD4 T-cell counts and overall survival.